Flow velocity underpins microhabitat selection by gobies of the Australian Wet Tropics

Water flow is a critical driver of aquatic ecosystem health and function. Amid rising concerns over changing flow regimes, there is an urgent need to understand the functional mechanisms by which flow influences patterns of freshwater biodiversity. We explored the functional link between flow velocity and microhabitat specialisation in a speciose group of freshwater gobies (comprising over half the total fish species richness) within insular streams of the Australian Wet Tropics under base flow conditions. We addressed two particular questions: (i) What is the relative selectivity of species towards streambed composition and water flow velocity? and (ii) Can patterns of microhabitat occupation be explained by differences in intrinsic flow performance among species? To answer these questions, we combined visual field observations of microhabitat use with flow tank assessments of flow speed performance. Tropical freshwater gobies displayed strong specificity towards flow velocities, while being relatively non-selective towards streambed composition. At opposite extremes of the spectrum, we found Sicyopterus lagocephalus occupying high-flow (>1.0ms^-1) microhabitats while Redigobius bikolanus selected slower-flow (<0.05ms^-1) areas. These patterns of microhabitat flow specificity were largely explained by the different abilities of species to swim and/or cling to the substratum under these different flow settings. Our findings confirm suggestions that predictable base flows in tropical streams support habitat specialists, which include one species capable of occupying areas of extremely high flow that very few other fishes can withstand. The functional link between flow and gobioid fish distribution patterns could occur throughout tropical streams of the Indo-Pacific and Caribbean as a widespread phenomenon that may help inform stream flow management guidelines to maintain this substantial component of tropical freshwater biodiversity around the globe.